In today's industry, success or failure depends in part upon knowing the up-to-date status of various assets. For example, in the freight delivery business, up-to-date knowledge of the location and, in some instances, the environment of various assets, such as pallet goods, is critical to efficient and reliable operations. Failure to maintain up-to-date status information can result in temporarily lost assets, sub-optimal use of the assets, and in the case of freight delivery, missed or late deliveries.
Recently, technologies have been developed that greatly assist in tracking locations of assets. For example, global positioning systems (GPS) use wireless signals transmitted by earth-orbiting satellites to calculate the position of a receiving device. Although relatively expensive, GPS receivers are capable of providing relatively accurate location information for virtually any point in the world.
More recently, radio frequency identification (RF or RFID) systems have been developed in which tracking or transmitting devices, often referred to as “tags,” wirelessly communicate with readers. RF tracking systems are typically used in parcel tracking and sorting, container tracking, luggage tracking, retail tracking, warehouse tracking and inventory operations. The tracking devices may be either passive or active. Passive devices absorb signals transmitted by the reader and retransmit their own signals, such as identification information. While a passive device does not require a local power source, its resulting transmit range is relatively short, typically less than 1-2 meters. In contrast, an active device, which sends a signal to indicate its location, includes a local energy source (such as a battery) that improves transmission range. Depending on the wireless signal system used by the device, the range may be on the order of several meters or several hundred meters, or even miles. Regardless of the types of device used, knowledge of the location of the device allows users to identify the location of an asset that has the tracking device attached thereto.
Although a tracking system is highly useful, there is a large initial start-up cost associated with these tracking systems; the larger the system, the more tracking devices are needed. In order to decrease the overall cost of the system, utilizing primary tracking devices (main tags) and secondary tracking devices (sub tags) to provide the same level of asset tracking and protection but decrease overall system cost has been proposed (e.g., U.S. patent application publication 2015/0296332 entitled “Asset Tracking System Having Primary and Secondary Tracking Devices”). In this publication, some assets being tracked are furnished with primary tracking devices and others with secondary tracking devices, which cost less and have less features than the primary tracking device. With a high aspect ratio deployment between the primary and secondary devices to lower the system cost, a 100% deployment rate can be achieved.
In some systems such as of the publication discussed above, the primary tracking device (main tag) acts as a coordinator and the secondary devices (sub tags) surrounding the coordinator act as end-devices. Each secondary device can communicate only with the primary device in proximity. Even with this, the number of sub tags surrounding a main tag at a point of time can be variable and unknown. If the number of sub tags is too high, it may cause communication collisions. An absence of a main tag can lead to a power drain of the sub tag, since the sub tag will continue to attempt to establish a communication with a main tag, even if one cannot be found.
There is a need for heterogeneous communication between the main tag and sub tags to overcome the problems described above, and other problems, as will become apparent to one skilled in the art.